Axially extending tie bolts are used in rotary machines, such as turbines used for power generation, to hold various components such as turbine stages, nozzles and spacers together on the core shaft of the rotor (or other components in the stator assembly). These bolts are typically circumferentially spaced about the rotor, with an annular rotor cavity or groove provided for accommodating the associated nuts.
The bolting that extends into cavities bounded by both rotor and stator components can significantly affect the flow of air in those cavities by pumping the air around the rotor circumference. This pumping action increases the air's tangential velocity if the bolts are on the rotor, and slows it down if the bolts are on the stator. Regardless of direction, the acceleration results in frictional heating of the gas, increasing its temperature as well as the temperature of adjacent components.
This so-called "windage" is undesirable, in that it uses energy from the machine thus reducing efficiency, and in that the increased component temperatures can limit part life. Several concepts have been developed to minimize windage by establishing a cover over the nuts and bolts, thus decreasing the pumping that will occur. The cover may be a separate piece, or it may be introduced by shaping the nut to present a segmented 360 degree surface to the cavity. This latter approach is used at one end of the rotor of certain of the assignee's land based gas turbines. More specifically, the nuts themselves form a part of the wall of the air cavity between the rotor and stator components. Each nut has a compound peripheral surface, a radially outer part of which conforms generally to the curvature of the rotor and which defines a pair of oppositely extending cantilevered "wings." A large hole in the center of the nut is threaded to receive the bolt in conventional fashion. Two smaller holes to either side of the central hole, i.e., below the "wings," are provided to reduce the weight of the nut. Even with this feature, however, each nut weighs slightly over 9.5 lbs.
The "wings" on the nut bridge the space between adjacent nuts, with the "wing tips" of adjacent nuts coming close to, but not contacting each other. Thus, the outer surfaces of the assembled nuts present a substantially continuous, smooth surface to the rotor-stator cavity of which they form a part of the cavity wall. The wall thickness of the nut varies in the different areas and this non-uniformity presents a variable stiffness, which in turn causes a circumferential variation in load transfer to the bolt. Similarly, the wings (that are essentially cantilevered from the sides of the nut) tend to put a tensile load into the sides and bottom of the nut, and a compressive load on the top of the nut, i.e., directly over the center hole.